The present disclosure relates to a sheet feeder for feeding a sheet to a predetermined processing position and an image forming apparatus with this sheet feeder as an automatic document feeder.
An image forming apparatus for forming an image on a recording sheet based on a document image on a document sheet includes an apparatus main body for performing image formation, an automatic document feeder and an image reading unit. A document sheet set on a document tray of the automatic document feeder is conveyed toward an image reading position of the image reading unit and a document image thereof is optically read. In an image forming apparatus using an electrophotographic process, a photoconductive drum is charged in advance and a laser beam modulated according to image data of the document image is irradiated to the surface of this drum to form an electrostatic latent image. A developed toner image is transferred to a recording sheet and then fixed to the recording sheet.
The automatic document feeder includes a pickup roller for feeding a document sheet set on the document tray. This pickup roller is pivotally supported on an arm member, retracted to a position above a sheet conveyance path (retracted position) before the document sheet is set on the document tray and lowered to come into contact with the upper surface of the document sheet and feed this document sheet (feeding position) after the document sheet is set. In such an automatic document feeder, it is known to change the posture of the pickup roller between the retracted position and the feeding position by coupling a drive shaft of a feed roller and a roller holding member for holding the pickup roller by a coiled spring (prior art 1). The coiled spring is mounted on the drive shaft with a constant holding force and includes an engaging portion with the roller holding member and transmits a torque of the drive shaft to the roller holding member to pivot the roller holding member. On the other hand, when the retracted position or the feeding position is reached and the pivotal movement of the roller holding member is prevented, the holding force of the coiled spring on the drive shaft is released and the coiled spring and the drive shaft are in a slip state. Accordingly, the torque of the drive shaft is not transmitted to the roller holding member. Further, there is also known an apparatus for performing a standby operation to move a pickup roller when an automatic document feeder is turned on to keep the pickup roller on standby at a predetermined position (prior art 2).
A user of the image forming apparatus often aligns document sheets using the upper surface of a housing of the automatic document feeder when setting the document sheets on the automatic document feeder. Vibration produced on the housing by this aligning operation could lower the pickup roller at the retracted position. This depends on the fact that the pickup roller is maintained at the retracted position with a relatively small force by a coupling member such as the coiled spring as in the prior art 1. If the pickup roller is lowered, a space where the document sheets can be set becomes narrower in a height direction and there is a possibility of problems that document sheets are rolled up when being set and the leading ends of the document sheets are less likely to be aligned in a sheet feeding direction to cause a multi-feed problem.
In the apparatus of the prior art 1, the problem of lowering the pickup roller can be solved if a coiled spring with a large holding force is used. However, a torque required to pivot the arm member increases, leading to a necessity to increase the capacity of a motor for pivoting the arm member. Further, in the apparatus of the prior art 2, the pickup roller is kept on standby at the predetermined position when the automatic document feeder is turned on, but the problem of lowering the pickup roller after the automatic document feeder is turned on cannot be handled.
An object of the present disclosure is to provide a sheet feeder and an image forming apparatus in which a sheet to be fed can be reliably set on a tray and fed to a predetermined position without problem.